text:
What should I look for in weather reports in order to pick a day for glider flying?
I understand weather can change unexpectedly, and local sky condition (e.g. cloud types) can't be predicted, but if I want to fly glider at a specific location, say, some time next week, and use weather forecast to pick the best possible day, what should I look for? e.g. "Cloudy" vs. "partly cloudy" vs. "overcast" vs. "sunny", is one preferable to another? (I wish the forecast would say something like "Cumulus clouds abound")

What kind of glider flying do you want to do? Soaring? Long distance? Mountain wave?

This question can not be answered. There are several types of glider flying such as upsloap and thermal which have different weather requirements. The geography may also play a part such as upslope soaring.

Just short glider flying lessons in/near mountains/valleys.

For thermal soaring, which is the majority of the time in the sport in North America, the best soaring days are usually the day or two or three after the passage of a cold front with high pressure behind it, where light to moderate winds are forecast.  You want relatively unstable air without any inversions, no mid to high level clouds, and low humidity but preferably enough to form cumulus clouds to mark the thermal tops (if the air is really dry you have no clouds at all and have to troll randomly looking for lift, or look for thermaling birds, or look for other gliders that have found lift). In other words, it's those clear sunny days after a high moves through, the temperature cools off, and you get those nice puffy widely separated cumulus clouds and light to moderate winds.
If a front ahead of high pressure has a strong pressure gradient behind it, it's likely that the winds the next day will be strong and this is bad because it tends to break up the thermals.  In that case I would plan to go the day or two after as the high moves in and the winds die off.
There are a lot of other factors, like the ground moisture content (wet ground sinks heat and inhibits thermal formation), but as a way to try to judge the best likelihood of good soaring conditions, the 2-3 days following passage of a cold front rule is the most reliable.
If you're not much at interpreting weather maps and can only go by the public forecast, and have to pick a day in advance, try to pick one of the first days following the day with storms and a temperature drop in the forecast due to a cold front, where it shows sunny or "partly cloudy" (which will usually be be desirable convective cumulus) for a few days after, and avoid really windy days which if it happens is typically the first day after frontal passage.
Exceptions to this are desert areas which have sunshine and good convective conditions for thermals pretty much all the time, and places like Florida where during any kind of sunny weather there is always convection (there is a localized thunderstorm somewhere in the state just about every day in summer).

Thanks. You have precisely addressed my question.

It very much depends upon what type of flying you normally do.
I can only really comment on what I do, you will have to adjust for your own circumstances.
At my local glider field we mostly fly on ridge lift as we have a very wedge-shaped ridge right next to the airfield, it has a very steep southwest face that tapers away towards the North. We also have a small range of hills to the SW of the field that faces NW that gives ridge lift. This means that I predominantly look for wind direction and strength, wind from the West gives good lift but from the East the wind just makes massive sink. We had a wind from the East for a couple month earlier this year and so all we managed was circuits.
The second most important lift for me is wave. I live in Scotland is quite small and which has mountains all down the western side of the country. I keep an eye out for wind coming off the sea and striking the mountains as this causes wave which ripples all across the country.
Cloud is obvious. I look for a decent cloudbase, high enough to allow me to fly without entering the cloud. Also I check for rain forecast and avoid rainy days. Cloud cover is less of an issue to me as I live in a hilly country where thermalling is less important than hill lift as there is always a hill nearby.
The best advice I can give is to talk to your chief flying instructor at the club and get advice specific to your airfield. I'm guessing that from the question you are a beginner flyer and so your flights will likely be around your airfield. Your CFI will know the area intimately and can tell you just what to look for and what websites to go to for forecasts and how to interpret them.


text:
Why do airplanes use AC electric power?
Primary electric power on larger airplanes is provided as AC. What aspects of aircraft make AC a better choice than DC?

if you search for "dc ac spike" or "dc ac Voltage spike" you will see that this property alone, from the AC power, can be enough to justify the adoption of it against the DC.

@user2485710 I have no idea what you're talking about.  Care to share a link?  You can put voltage spikes on any power line, it doesn't matter if it's DC, AC, or 12-phase

@NickT yes, but AC reacts way better than DC. ask here http://electronics.stackexchange.com/

@user2485710 I have over 9000 rep on that site and am an electrical engineer.  I don't know what you're talking about.

@NickT in short handling a spike in a DC system it's more expensive and less reliable than what an AC system can do.I lost the links to the articles that were explaining this.For example in your house you probably have AC,your pc is probably connected to a power surge, to protect your PC from a spike in a DC world you would need to dissipate the current that is the "extra" voltage from the spike as heat, with AC you have a lot of cheaper and more reliable options http://en.wikipedia.org/wiki/Surge_protector .People use lightning rod wired to the ground against lightnings not giant heaters.

Something interesting (to me at least) is that in the Dassault Falcon aircraft, they eliminated the AC bus in newer aircraft and generate 100% DC.  Where AC is needed for specific instruments, they replaced the instruments with versions that had built-in inverters.  So I wouldn't assume that all airplanes are using AC.  Perhaps (just a guess) larger airplanes are going to AC because of the longer wire runs?

@user2485710 those surge protectors are also dissipating the excess voltage as heat by using MOVs or other non-linear devices.  TVSs, for example, are used on DC power lines (and data lines) fairly frequently to suppress everything from ESD to heavy-truck alternator load dumps which can shove >100 VDC onto an automotive DC bus.

I haven't seen anyone here mention the interesting artifact of "DC dies and AC lies" -- that is, DC powered instruments tend to drop to zero when power fails, AC instruments (like the torque gauges on the King Air) freeze wherever they were last, which is NOT a very friendly failure mode. Incidentally the King Airs have all of: 28V DC, 28VC AC, and 110V AC.

Adding to @Lnafziger 's comment, I think the only advantages of using AC is the easiness of generation and of voltage conversion (and good quality of the converted-voltage current). Else transporting AC is not as efficient as DC (due to reactive power), and as a matter of fact is not used on very long lines (like inter-country HVDC lines). Many electronic devices need DC (albeit at low voltage).

@mins HVDC lines are not used for distance reasons. They are used because it is very hard to synchronise two country's AC phase. True, you may find HVDC used for under water cables for distance reasons... But most planes spend very little time under water.

AC is easier to produce with the engines, that act as generators.
The engines have a rotating shaft that it is easily equipped with magnetic dipoles all around.

Then, depending on the instrument, the current is either used directly or, by the use of converters, in DC form as not only it is easier to produce, it is also easier to convert. So electronic devices that uses 12, 5 or 3V may contain their own voltage converter (though this is less and less true with modern efficient switch-mode converters). If DC conversion was as much efficient as AC conversion, we wouldn't see high-voltage lines. Last but not least, it's easier to switch, because the current is null twice per cycle. DC must be switched at full current, which is expensive and weight costly. (thanks mins)

Not entirely correct, I think. The AC needs typically be 400 Hz constant frequency, regardless of how fast the engine is spinning. So the engine and the generator are connected with a Constant Speed Drive unit, which in essence an automatic transmission that keeps the generator at constant speed.

@Hilmar I left that out on purpose, it is not required to understand "why AC instead of DC".

@Hilmar That's not always true-some aircraft have variable "wild" frequency power 350-800Hz depending on engine speed.

@Federico: Except it's kind of essential. There are no DC generators, only AC generators with converter. But when there is a converter, the frequency does not matter. When there is not, the frequency and the phase must match, which is quite a bit of complication.

That's an inefficient diagram you have there, 2 south poles, 4 north poles on the rotor!

@JanHudec It is perfectly possible to produce DC without producing AC and rectifying it. The device called a dynamo; early electrical distribution systems used these and supplied direct current. This is where the whole "War of the Currents" came from.

@DavidRicherby, actually, yes, it is possible to produce DC without producing AC first, but not with a dynamo. In dynamo the commutator that rectifies it is directly on the shaft, but in the windings the current is still alternating. Batteries, fuel cells and homopolar generators do generate DC directly.

@JanHudec I think that claiming that a dynamo has AC internally is being unnecessarily pedantic. A dynamo is a device where the only way of connecting to it gives you DC. It may well have alternating currents internally but there's no way to connect to that. It's not meaningful to describe a dynamo as being an alternator connected to some kind of rectifier, since you can't separate out those two components.

@DavidRicherby, but it is exactly what I was meaning right from the start. Since dynamo is exactly the same as alternator except it uses commutator in the output, it would not make anything easier (well, it would; dynamos wouldn't have to be phase-synchronized).

@DavidRicherby, anyway, since dynamo and alternator are essentially the same except for the connection, this answer does not actually explain the choice at all.

I wish people would explain why they downvote.

It's easier to produce, and it's also easier to convert, so electronic devices that uses 12, 5 or 3V may contain their own voltage converter (though this is less and less true with modern efficient switch-mode converters). If DC conversion was as much efficient as AC conversion, we wouldn't see high-voltage lines. Last but not least, it's easier to switch, because the current is null twice per cycle. DC must be switched at full current, which is expensive and weight costly.

Downvoted because it is not how it is done. AC generators are very seldom connected directly to the engine distributing varying frequencys to the rest of the plane. The normal way is for the generator to produce DC (often enough by rectifying an AC generator). The DC is then converted into a stable 400Hz AC to feed some equipment. In a large plane you might find quite a few electrical buses, some 400Hz, some 60 or 50Hz, different DC voltages.

@ghellquist please have a look at the comments above. You'll see that other users say that there are no DC generators. who should I listen to?

@Federico: part of this confusion is language usage. English wikipedia has this description. I cannot say whether it is right or wrong: "Terminology: Electromagnetic generators fall into one of two broad categories, dynamos and alternators.

Dynamos generate pulsing direct current through the use of a commutator
Alternators generate alternating current". If we use this description, in the World War II most aeroplane generators where dynamos, creating a pulsating DC. In order to create AC there would be an inverter (motor + alternator).

@ghellquist then why did you downvote? I am not speaking of WW2 planes, but modern larger airplanes, as asked in the question

What the other answers have failed to note is that on a plane, its not just AC power, but 3 phase AC power.
Depending on how the plane is wired up, you will either get the benefit of reduced cable weight or higher reliability (or some blend of the two).
Delta
If the plane is wired up with a Delta transformer, then 3 wires are used to carry the electricity. However, what is crazy is that the Delta transformer (or alternator) will continue to run (albeit at a lower capacity) with one of the windings destroyed.
Wye
In a balanced wye configuration the 4 wires come from the transformer. Each of the first three wires carry current from the transformer, with the 4th as the common "return". However since each of the wires are "balanced", the actual current on the common is approximately zero (and not the 3 * current of the output). That means your 3 "live" wires only needs to be thick enough to carry the current one way. (typically with DC you would need to have wire thickness to carry the current there then back again). So in effect we can achieve the same power transmission with half the weight of a DC setup.
Since on planes, both redundancy (safety) and weight (economy) are both massive factors in cost. It is actually economical to run power on AC, and convert to DC where necessary.

The size, weight, and cost of switchgear for DC-DC conversion at any given power is much higher than AC-AC and AC-DC conversion. So whether a piece of equipment requires AC or DC, it can be converted more easily from an AC source than from a DC source - but more importantly, it can be done with less weight, volume, and cost.
It's the same reason 400Hz is used rather than 50Hz or 60Hz - weight.  A generator or transformer that can handle a given load is physically smaller and lighter at higher frequencies due to issues with core saturation at lower frequencies.  A smaller, lighter core can be used for higher frequencies.
While switchgear has evolved over the decades, and weight and cost aren't as big an issue as they were in the past, the generator itself still has to have a stator and windings, and these are still physically smaller and lighter for a given power output than the equivalent DC, or low frequency AC generator.

Since DC generator is just an AC generator with converter, it can be made from high-frequency one and thus have the same low weight. It is even advantageous, because less stabilization is needed.

@JanHudec "DC generator is just an AC generator with converter" I suppose you can make it this way, rather than using a real DC generator. But why?  An AC generator without an AC-DC converter is still lighter and cheaper than an AC generator with a converter.  Further, what are you going to do with that DC? You're going to have more expensive and heavier DC-DC and DC-AC converters throughout the plane. AC-DC and AC-AC converters are still cheaper and lighter.

@AdamDavis: You'll need some conversion electronics by the generators in both cases -- the rotation of the several engines that may provide power is not synchronized, so just feeding the raw output from each AC generator into a common bus wouldn't end well anyway. (And in contrast to terrestial power plants, producing electricity is not the main task of a plane's engines so you cannot just adjust the engine speeds to lock frequency between the generators).

There is no "real DC generator". Electromagnetic induction only works with changing magnetic fields and thus produces changing current. So a DC generator is either wound generator with commutator instead of two separate rings or an induction generator with diode converter. There is no reason why it couldn't run at the same rpm as the AC generator and consequently no reason to be heavier.

@HenningMakholm That's true, particularly on modern and larger multi-engine planes. The reason we do it today is largely historical. Lots of planes now have DC and AC buses, multiple generators, buses, and converters that will power one or more buses from the others depending on circumstances. They could change the systems completely over to DC, but it would require a significant change in the industry to justify the cost. We are now building planes based on the decisions made years ago due to the weight difference of direct AC and DC generators

@JanHudec I understand what you're saying, however all of my reading suggests that isn't the case.  If you have additional technical references showing that DC generators are as light as AC for a given power output, please update http://en.wikipedia.org/wiki/Utility_frequency#400_Hz at least, and let the Us government know one of the basis of MIL-STD-704 is incorrect. The DC/AC question has more to do with historically easier/cheaper/lighter power conversion at individual points along the plane for specific historical reasons. But my research suggests AC generators were lighter as well.

@AdamDavis: That paragraph does not compare to DC, only to lower frequency AC. The point against DC is obviously that it can't be transformed.

Does it have to be aircraft-specific reasons? I would imagine the reasons are not necessarily very aircraft related:

Different subsystems need different working voltages internally, and supplying AC makes it easy for each component to transform down to whatever it needs.
Where mechanical power is needed, asynchronous AC motors are a lot simpler (especially in terms of maintenance requirements) than DC motors.
Engineering inertia. All kinds of components already come expecting AC; even if DC is in isolation superior for some uses (as it might well be for electronics, with today's solid-state voltage converters) supplying DC to aircraft systems would mean that you need to have a DC supply network in addition to the AC supply for those components that are not yet available and certified in DC versions. That extra wiring alone might easily offset the advantages of having some of power supplies be lighter.

I assume that aeroplanes use both AC and DC for powering the various subsystems:
If energy has to be stored in accumulators - well, they only store DC current. But
if generators are involved you won't have a chance of totally avoiding AC since a rotating movement is essentially sinusoidal:

If you put a rotating magnet into a coil it will produce an alternating magnetic field in the coil => the coil will produce AC
and if you put a rotating coil into a magnetic field the field will alternate between going through the coil in one direction and in the other again producing AC current.

One can now put the AC through a rectifier or use a wiper (a switch that can reverse the current and is mechanically triggered twice at every revolution of the generator). But if you rectify a sinus you still get a voltage that goes up and down, only it no more passes the zero level. This means you will sometimes draw too much energy from the generator (when the voltage is high) and at every revolution the voltage will shortly drop to zero meaning you won't be able to draw any energy at all.
If you don't constantly draw energy from a generator, the generator will vibrate.
You wouldn't like that in an aeroplane but there is a remedy:
Using three-phase AC: You can generate three sine waves with the same generator using coils that are a third of a revolution apart. When the voltage from one of these coils drops, the voltage at the two other coils rises - and the power produced (which is linear to the voltages squared) will add up to a constant flow.
The next thing one has to know that if you want a constant stream of power from a motor the same reasons make you use three AC waves here, too.
Normally this AC is generated from a DC voltage, though, because you might want to adjust the frequency the motor is running at or the amount of power it provides hence you want to adjust the frequency of the voltage the motor gets.

This goes back to the 1930-s and has been kept because it was a good solution and still works.
In the 1930-s the big electronic revolution started and accelerated in the second world war. This is of course before the transistor, so vacuum tubes where used in radios, radars and so on. The generators on the engines were DC-generators and feeding batteries (remember, no diodes available for AC generators). But the vacuum tube elecronics boxen needed several different voltages inside: something like 6.3V for heating and several other voltages, say 12V and 400V. In order to produce these transformers were the best solution. But transformers need to be fed AC.
To create AC an inverter were used. In those days, a DC motor connected to an AC generator. The RPM of the DC generator was, somewhat, regulated to get in the ballpark correct AC frequency.
As the weight of a transformer is, somewhat again, inversely proportional to the frequency you would want a high frequency in order to keep weight down. As losses increase, again somewhat, proportional to frequency, you would want to have a low frequency. Somewhat in the middle of frequencies the engineers, who incidentally knew exactly what the were doing, landed on 400Hz as a reaonable compromise.
And now we keep on using 400Hz, simply because it works. All of the components and lots of the engineering decisions has changed, but 400Hz AC still is a good in the middle compromise.
You might be interested to check this manual: b24 electrical system or
b17 electrical system

A transformer that converts the same power on a higher frequency is much smaller and lighter.
A modern design of the compact isolating power supply (say 220 V 50 Hz to 12 V DC) would often include a frequency converter, then a high frequency transformer, then a rectifier (a big 50 Hz transformer is now uncommon). But maybe the standard has settled before this complex electronics became available, or maybe because higher frequencies may be source of interference.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
Why are adjustable area jet engine nozzles mostly limited to military use?
It seems that only jet engines with afterburners use adjustable area nozzles (this means adjusting the area of the nozzle but not necessarily the direction). This includes most fighter jets, as well as bombers like the B-1B and Tu-160. Outside of the military or afterburning engines, they don't seem to be used, even for smaller engines.
What is the reason for this? Is the Variable Area Fan Nozzle (VAFN) concept related?
Wikipedia explains that it is related to supersonic operation; I'm looking for a more detailed explanation about the reasons for this.

A variable area nozzle helps to adjust the exit pressure of the combustion gases to ambient pressure. When they leave the turbine, the combustion gases have still some remaining pressure which is converted into speed by a convergent contour of the nozzle.
If the combustion gases have enough pressure to be accelerated to supersonic speed, the nozzle is actually first convergent and then divergent to achieve the best acceleration of the flow. The convergent, subsonic flow accelerates until in the section with the smallest area, called the throat, the speed of sound is reached, and the following divergent section accelerates the now supersonic flow further until its pressure drops to ambient pressure. This con-di-nozzle needs to adjust both the cross section of the throat and the cross section of the exit area. Failure to get especially the throat area right will mean substantial thrust loss in operation.

Afterburning engines need adaptable nozzles due to the different operating conditions in dry and wet mode, so they profit the most from adjustable nozzles. Reheating the exhaust gas means to increase its volume, so the nozzle needs to become wider to be properly matched. In general, if the exit speed of the combustion gases is supersonic, an adjustable nozzle is required. Even some early jets without afterburners had an adjustable nozzle: The central cone of the Jumo-004 nozzle could be moved fore and aft, which changed the location of the bulbous aft section, helping to get the throat area right. Note in the picture below that the cross section widens again in the last part of the nozzle, which shows that the exhaust stream of the Jumo engine was mildly supersonic.

Airliner engines would only gain little from an adjustable nozzle, and considering the mass increase of a fully adjustable nozzle, would in total have lower efficiency. Short-range aircraft usually have nozzle geometries optimized for field performance while long-range aircraft prefer to optimize for best cruise efficiency. Their core flow exit speed is still subsonic or only mildly supersonic - please note that the speed of sound in the heated combustion gas is much higher. At 500°C the speed of sound is almost 560 m/s.
The variable area fan nozzle tries to do for the fan flow what the regular nozzle does for the core flow of a jet. So this is indeed a related concept and helps to optimize performance both at low and high speed. The picture below is copied from the US patent 2011/0302907A1 and shows an adjustable end part of the cowling (crosshatched, 54). The exit speed of the fan flow is subsonic at take-off and mildly supersonic in cruise, and since its mass flow is much higher than that of the core in airliner engines, making the nozzle variable pays off earlier. By adjusting the exit pressure by varying the exit area it can squeeze out some performance gains.

Correct me if I'm wrong but the supersonic Tu-144 didn't have a variable nozzle because it was designed around a certain cruise speed. The nozzle shape was most effective at that speed and since the aircraft didn't spend much time flying outside of that speed, nothing more complex was needed. Same principle for subsonic jetliners: they are optimized to fly at a certain speed, so they don't need that much flexibility.

@L3R5: You're right, I never noticed that the RD-36 had no variable nozzle. The NK-144 used on earlier Tu-144s, however, used a regular con-di-nozzle. I suspect that the RD-36 could shift the central cone forward and backward to adjust for dry and wet states, but I could find no information about this.

In figure 1.15, why is pressure decreasing with velocity increasing before the nozzle / choke point? This goes against everything I know.

@Pheric: The pressure must be the static pressure. Of course, total pressure should be constant, apart from viscous losses. For subsonic flow this makes sense.

While not an engineer, I do fly aircraft with adjustable nozzles, and I may be able to shed some light on their basic principle of operation:
Purpose
The purpose of the nozzle, in subsonic flow, is to constrict the airflow, thus greatly increasing the thrust as it leaves the engine (technically speaking, it converts the gas into a thrust, thereby making the gas generator a jet engine.)  If there was no nozzle, then the gas leaving the turbine would not have nearly the same thrust coming out of the back as it does when the nozzle restricts.  You can think of it like placing your thumb over the end of a hose, the flow is much faster as you restrict the flow of water.  More technically, flow speed increases while the actual pressure decreases--the venturi effect.  However, during afterburning operations, the nozzles have the reverse effect, and closing them actually reduces flow speed.  Thus, during supersonic flow the nozzles actually open.
Operation
Moving the throttle forward on a jet does a few things, it increases fuel, creates more gas, and also restricts the nozzles.  During taxi operations, I'd guess that the nozzles restricting, as you come off the idle stops, have more of an effect on the jet moving than the actual increase in N2.  In fact, were the nozzles not to restrict properly and suddenly open during flight, the jet would fall out of the sky.  Its an actual emergency we practice, because, occasionally, nozzles will malfunction which, in a worst case scenario, can leave you with two useless engines.  However, as with most things, there's a caveat.  When operating in afterburner the nozzles open back up again.  Nozzles that are stuck closed in AB have the same effect that nozzles that are stuck open in mil do--you become a brick.  So essentially, nozzles are only required if your engine is capable of afterburning and generating supersonic flow.  You can't generate thrust with open nozzles below burner, and you can't generate thrust with closed nozzles in burner.  Its just more complexity that non-AB engines don't need, and more that can fail in flight.
Efficiency
If there was any reason why commercial aircraft do not have the same engines we do, this would be it.  With the exception of highly inefficient turbojet engines, nearly all commercial aircraft are equipped with turbofan engines, high-bypass turbofans to be precise.  Without getting into the weeds, the high-bypass flow used in modern engines allows them to operate much more efficiently than our engines.  With a high-bypass engine, all flow will remain subsonic, and a fixed exhaust is not only more economical, its also safer because it can't open and reduce thrust.  Fighter aircraft also use turbofans, however, ours are low-bypass, and consume much more fuel than our civilian counterparts.  The benefit is that we are also able to operate in a much more diverse environment: high AoA's, supersonic flow, rapid power fluctuations, etc...  Given our mission the trade off between lost efficiency vs performance gain makes sense. For reference, on the Super Hornet, our engines burn over 38,000pph at MAX.  This kind of inefficiency would quickly bankrupt an airline, because they just don't have the same kind of operational requirements.  Granted, we don't fly around at MAX all the time, but you get the idea.

+1 for "Its just more complexity that non-AB engines don't need, and more that can fail in flight." Obviously, that's a huge consideration, both in terms of cost and in terms of reliability in something like a passenger airliner. Additional single-points-of-failure are generally avoided when there isn't a particularly compelling reason to add them.

Peter Kämpf already gave good long answer with nice diagrams. tl;dr, the thing boils down to subsonic and supersonic flow behaving differently. The goal is to increase the exhaust speed. For subsonic flow you achieve that by restricting it, but for supersonic stream you have to expand it. So supersonic engines (and supersonic exhaust speed is rarely reached without afterburner) need variable area nozzle, subsonic ones (all airliner engines except Concorde) not.

Don't forget the Tu-144. It needed the afterburners even more badly than the Concorde; without them it would not sustain supersonic speed.

Seems to me your own link gives the answer:

Non-afterburning subsonic engines have nozzles of a fixed size because
  the changes in engine performance with altitude and subsonic flight
  speeds are acceptable with a fixed nozzle. This is not the case at
  supersonic speeds as described for the Concorde in section "Nozzle
  area control during dry operation".

That adjustable nozzle provides thrust vectoring. This is most effective at the rear of the plane far from the center of mass, where they provide assistance with pitch and yaw.
So with the under-wing engines it would only help with roll. Pitch and yaw are not helped in that configuration and the existing ailerons are more than enough for that.
The mechanism is a complex and heavy mechanism that needs maintenance and extra fuel to bring along.
Most (all since the concorde) commercial aircraft are subsonic fliers where the change in atmospheric conditions doesn't change the effect of nozzle area enough to warrant the added complexity.

I'm asking about the area change, not the directional change, I edited the question to hopefully make that more clear.

So you ask, "Why do jet engines with afterburners use variable area nozzles?"
Such engines might be for military or commercial use, but are used to propel the aircraft to supersonic.  The Concorde was an example of a non-military supersonic aircraft, the engines on which were afterburning.
There is one basic reason why the nozzle area changes, and it is "to maintain engine airflow."
Nozzle Performance for convergent nozzle:
A nozzle performance plot shows the flow parameter -- Wg·Sqrt(Tt)/[Pt·A] -- versus nozzle pressure ratio -- Pt8/Ps8 where:
    Wg = airflow, plus combustion by products, hence vitiated air
    Tt = total temperature
    Pt = total pressure
    A = area at nozzle throat
    Pt8 = total pressure at the nozzle throat
    Ps8 = static pressure at the nozzle throat
reaches a maximum at M = 1 (nozzle throat Mach number = 1).
The value of the flow parameter is around 0.5318.
WgSqrt(Tt)/[PtA] = f(M,k,R), where
k = specific heat ratio,
R = gas constant
Without Afterburner
During dry operation at 'military power', the engine is at it's design point, at 100% compressor speed, & turbine design point if the compressor & turbine are designed to have a good match point (thermodynamic & aerodynamic).
At this point the flow parameter at the nozzle throat is at its max ~= 0.5318.
With Afterburner
If the afterburner is started, then the total temperature into the nozzle, Tt8, increases sharply.  Suppose the temp doubles?
then Sqrt(2·Tt8) = 1.4·Sqrt(Tt8), and you have 1.4 times the choked flow parameter at the throat.
But the flow parameter cannot increase, so what happens is that the airflow decreases by a factor of 1.4.  And just where does this airflow come from?  From the turbine exit, which comes from the burner exit, which comes from the compressor exit, which comes from the compressor inlet.
What happens next?  An engine surge if the FADEC (Full Authority Digital Electronic Control) tries to maintain compressor rotor speed by increasing fuel flow to the burner in order to increase turbine output.  Actually, this could result in a turbine over-temp, so the FADEC has to cut back on the demand.
As well as lighting the afterburner causing Tt8 to increase, it causes a decrease in Pt8 ... there is a drop in pressure that is due the heating.  This is a Rayleigh process, and accounts for the 'hot losses'.  In an A/B (afterburner), there are frictional or cold losses, but these are minimal in comparison to the hot losses.  These total pressure losses also cause the flow parameter to increase, as Pt is in the flow parameter denominator, and contribute to the loss in airflow.
The solution to the problem of higher Tt and lower Pt that restricts nozzle flow is to increase the nozzle area -- then all of the engine's other components are happy.  Also happy is the pilot who is chasing or being chased by an adversary, and who doesn't want a missile fired up his tailpipe because his engine quit on him.
About the divergent section of the nozzle - it is to expand the gasses to near ambient, and obtain some more thrust in doing so.

Welcome to aviation.SE! You might consider formatting your answer to make it a little easier to read.

Also, the result of an afterburner is much higher thrust, but the cost is much more fuel.  So it pays to fly subsonic.

"A" for engineering, but "C-" for English. Paragraph breaks work well in this software, and will make the answer readable, which presently it really isn't. Please use the "EDIT" button to add paragraphs. Thanks!

I have attempted to organize this wall of text presentation.

One thing so far not mentioned or considered is that as the jet pipe nozzle changes area the static pressure within it will change. Consequently the differential pressure across the L.P. turbine (assuming a twin or triple spool engine) will also change. This speeds up or slows down the L.P. compressor.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How often does fan blade out happen?
Jet engines are designed to contain a fan blade failure, and the engines and airplanes are designed with this type of failure in mind. This is more critical in the modern high-bypass designs with large fans. Pictures from bird strikes sometimes show pretty severe damage, but I don't remember any that actually lost a fan blade.
How often does a jet engine actually lose a fan blade?
This is different from a rotor burst, which is uncontained (like with Qantas Flight 32).

According to Airbus 2007 Flight Operations Briefing Notes, Supplementary Techniques, Handling Engine Malfunctions The failure rate of aircraft engines has reached an all-time low. This means that many  flight crews will never face an engine failure during their career

I found the following rather surprising statement in a (probably quite old) University of Southamption document

Rolls-Royce has never had a service failure of conventional fan blades in over 40 million hours of operation, and there have been no service failures of wide-chord blades in over 10 million hours of operation (Baldwin 1993).
In one incident, RB211-535E4 engines on a Boeing 757 were struck by a flock of Canada geese near Chicago – some seven birds, each around 3 kg, were ingested.  The wide-chord fan blades withstood the impact, which was some eight times greater than the requirements for certification, and the engine did not have to be shut down in flight.

So far as I know, bodies like the NTSB don't collect statistics on blade failure events.

Fan blade failure isn't unknown.  Three widely reported incidents:
1989 January 8,
British Midland Airways 92.
A fan blade fractured due to aerodynamic flutter.
2016 August 27,
Southwest Airlines 3472.
Part of a fan blade broke off, after the engine ingested debris.
2018 April 17,
Southwest Airlines 1380.
A fan blade separated at the root.
Page 4 of a 1997 FAA advisory circular quotes an SAE report of
676 uncontained engine failures from 1969 to 1997, but doesn't discriminate between blade failure and other failures.
Page 19 of a 1991 SAE Report on aircraft engine containment may have the closest answer:  high bypass turbofans had 1.12 events per million hours, and 0.56 when excluding fan blade events;  subtracting,
we get 0.56 fan blade failures per million hours that resulted in uncontained engine failure, for commercial transport during 1976-1983.  Blade failures that did not cause uncontainment aren't counted here, so the true number should be somewhat higher.  This hundred-page report goes into eye-watering detail.

The delta 1288 does not qualify, because the question explicitly excludes cases where the hub (disk, rotor) fails. Most failures from the advisory circular also probably don't qualify as a fan blade failure is supposed to be contained—the two Southwest accidents are such serious issue because a mere blade failure caused failure of the cowling that was designed to survive it. Unfortunately the last link does not work for me. Are you sure you are counting just fan blade and not all fan failures?

Last link simplified - does that work better?
Their wording is "non-contained rotor failures", including or "excluding fan blades."  Yes, blades.

But, single blades, or all fan blades, including piece of the blisk they were in? It's not clear from that wording.


text:
Where did the standard pressure 29.92" come from?
Below the transition altitude (18,000 feet in the US), pilots use local pressure readings to calibrate their altimeters.
Above the transition altitude, pilots set a standard pressure to calibrate the altimeter. The standard pressure is 29.92 inches of mercury or 1013 hPa.
Where did this value come from?

Good question. Why use inches of mercury, makes about as much sense as burnt villages per meadow.

@Koyovis, inches of mercury because the early barometers and manometers used a column of mercury to balance the measured pressure. You might still find the that type in medical offices in use for measuring blood pressure.

It is called "standard pressure" because 29.92 In-Hg (or 1013.25 hPa) is the standard atmospheric pressure at sea level according to both the ISA (International Standard Atmosphere) and the US 1976 Standard Atmosphere.
Below 18,000ft, local altimeter settings are used because you need to know how high you are above terrain, or whether or not you're at the proper altitude for an instrument approach, or at pattern altitude, etc. In cruising flight above 18,000ft, except for a few places in the world, you are well above any terrain, so errors in actual altitude of even a couple thousand feet is unlikely to cause a problem. In this case, having all airplanes use a common altimeter setting is useful because it 1. doesn't require you to change it frequently as you pass through changes in pressure, and 2. it helps ATC ensure separation without having to inform flights every 10 minutes of a new setting. It makes sense to use standard pressure in this case because it represents a good baseline average, and you may have noticed that reported altimeter settings tend to average around 29.92 anyway.
That being said, there are instances where you may still need to know the en-route altimeter settings for flight planning purposes. Suppose the minimum safe altitude along your route is 19,000 ft MSL and current pressure is 29.42. If you decided to cruise at FL190, you would actually be flying at about 18,500ft MSL, and would be below your minimum safe altitude. Colder than standard temperature can have similar effect as low pressure, and will result in the airplane being lower than the indicated altitude.

It is called "standard pressure" because at an altitude of mean sea level (mean of all tides low to high) averaging the air pressure across equator to pole, winter to summer, over land and sea and day to night you get an average pressure that will register on the barometer at around 29.92 inches of mercury.

Yours is the only answer that actually addresses the question in my opinion. The others answers address the usage of standard pressure, but not how it is arrived at.

"standard day" model of the atmosphere is defined at sea level, with certain present conditions such as temperature and pressure. But other factors, such as humidity, further alter the nature of the atmosphere, and are also defined under standard day conditions":
These are the numbers for a standard day.
1013.25 hPa (14.7 lb/ in2)  (pressure)
(T): 15 °C (59 °F)  (Temperature)
(μ): 17.3 µPa·s (3.62 × 10−7 lb s/ft2)  (viscosity)
(ρ): 1.225 kg/m³ (0.00237 slug/ft3  (density)


text:
Why is ditching speed higher than stalling speed?
I was looking for a checklist for the Cessna 182 and I stumbled upon this one. When I looked at the speed-table, I found an interesting entry. It says "Ditching" with a speed of 65 KIAS:

Everything I read and watched about ditching so far told me to ditch at the lowest speed possible.
Why should I ditch with 13 knots more than I could (Vso is 52 KIAS)? I can think of no reason I would like to do that.

I dunno, seems like it's probably so you don't stall-spin it into the water at 50 feet?

How will you flare if you're flying an approach at Vso? That seems like a rather bad idea for any sort of approach.

If you have to ditch a plane with fixed undercarriage at any speed, it's very probable that the plane will tumble over on contact, and you'll drown... In my opinion, with a light plane, it is better to approach at the best glide speed, and then skim the surface in ground effect, gradually lowering flaps and pulling the stick till it stalls, hitting the water with the minimum possible horizontal speed. Perhaps you are lucky enough to survive...

Because stalling speed is the speed at which the airplane stops flying and starts falling. When ditching the objective is to land the airplane int the water in a controlled manner, rather than having it fall into the water in an uncontrolled fashion. Thus, ditching speed must be greater than stalling speed. Same as with touching down on a runway - approach speed must be greater than stalling speed or else you're not "landing" - you're "crashing". A ditching is not supposed to be a crash.

The dynamics of landing on water are very different from landing on a runway, because water isn't "solid". Instead of using the dampers in the landing gear to kill the vertical velocity and then using the brakes to stop, on water you want the plane to "hydrofoil" on the fuselage and then slow down and float. (Of course landing a seaplane normally is different again, because the landing gear has floats).

This is somewhat restated and overlaps the other postings. Here are my thoughts on this:

The ditching speed is to provide a safe approach speed. Depending upon the seas, you may not have the ability to judge your altitude, and this avoids stall/spin issues when ditching. The reality is that this type of complication to an unfortunate situation happens frequently. There are Youtube videos of people stall/spinning in as they are doing a ditching.
Frequently over smooth water, and even rough water, the height above the water is difficult to judge. Even if you don't have a seaplane rating, find an instructor to give you some simulated glassy water landing practice. A good time to do it is when you have night ILS approaches.  You can even do it under the hood, and simulate a 0/0 approach.
Many ditchings are done with loss of power.  If you are faced with performing one, try to have some fuel left, because your A/C is more controllable with a little power.  From those who have experienced it, I have heard that they see the gear hit the water out of the corner of their eye (C182) or they feel the prop picking up spray (Arrow). Back elevator is a real good thing, as it may avoid flipping over, which can hurt you and be very disorienting. Also, I wouldn't compromise your ability to fly, but you might hold the yoke/stick differently, as the force of hitting the water has broken a few hands/wrists or caused dislocations.  That only hampers your ability to manage everything else.

Also for preparedness, consider the FAA underwater egress course.  After you've been dunked in a high school swimming pool a couple of times, it is easier to keep your wits about you when you plunk into a great lake with a 42F offshore water temperature mid-summer. Someone who has been there swears by the underwater egress course. (not me)
Also read up on what you can about ditching orientation.  Like where the swells are and how you are oriented to them.  It softens the bang.
A few other things: for great lakes crossings, the preflight briefing includes having passengers identify what bags, sleeping bags, or whatever that they are going to put in front of them if there is a ditching. It includes what to do if the plane gets inverted. Most GA aircraft do not have 5 point, or shoulder harnesses in all seat positions, so protecting faces is important. Also when doing a lake crossing I recommend that everyone have their life preserver on before feet wet. Because SAR at night on the water is problematic, I normally do not fly across large bodies at water at night in SE piston aircraft. Just too many factors multiplying and stacking against me.  Flashlights and strobes are nice, but having looked for people with strobes over open water at night, while raining, I can say that they were very very difficult to spot under only mildly adverse conditions.
Finally, let me address then shedding of 13k airspeed. At 52k Vso, the v**2 component is 2704, and at 65k it is 4225, so you are right, in that you can reduce substantially your kinetic energy of impact.  But given the likely lack of horizon, the difficulty and inexperience you are likely to have ditching, the handling characteristics of the aircraft at Vso, the downside of a stall/spin, and the possibly increased kinetic energy, the impact disorientation, etc. it is easy to see why the higher speed and better controlability of a more "normal" approach speed is beneficial.

As far as I'm aware - and maybe a seaplane pilot could give a better answer - one of the big issues with ditching is judging your height above the water. If you stall it in, you might get it wrong and lose control completely.
I checked the POH for a 182RG, which has a Vso of 39 KIAS and ditching speed of 60 KIAS. The ditching checklist gives descent rate as the primary consideration, not airspeed:

Power -- ESTABLISH 300FT/MIN DESCENT at 60 KIAS

In the amplified procedures section, it says:

Avoid a landing flare because of difficulty in judging height over a
  water surface

In other words, better to descend slowly under control until impact than stall it at an unknown height above the water. As Bob Hoover famously said, "If you're faced with a forced landing, fly the thing as far into the crash as possible".
I did a bit of Googling and found similar checklists for other aircraft, e.g. a Piper Arrow:

Power--ESTABLISH 300 FPM DESCENT AT 90 MPH

I tried to put this in as a comment to @Pondlife's answer, but ran out of room, thus this answer is an amplification of what he said. He is correct. Judging height above the water is a challenge, especially if you've not had experience doing that, which typically a ditching pilot will not have had. Wave action helps, but the problem there is that you may misjudge how big the waves are and thus will misjudge your height. Landing alongside a shoreline can be a big help.
When people get their seaplane rating, they're taught how to do "glassy water landings". In other words, landings on water that has no wind or current action to disturb the surface. That makes judging height difficult even for an experienced seaplane pilot if the water surface is all there is to look at. The procedure for a glassy water landing is the same as for ditching: maintain a constant rate of descent carrying power until the aircraft contacts the water, then cut the power and up elevator.
The same procedure is used for night landings on water, although in general one should try to avoid having to make such landings.

BTW: the sprinklers/bubblers below the diving tower in a pool used for competitive diving are there for pretty much the same reason: you just can't tell your distance from the water surface easily.

authors: Bob Jarvis - Слава Україні


text:
What are these wheel-chocks made of and what color do they most probably have?
I found a new hobby of mine: Colorizing black and white photos. My first project was a B-17 and now I'm working on a He 162 Volksjäger. The plane is on ground and fixed with wheel-chocks.

Now the question is what they are made of and what color they could have. Maybe there is a specialist for german aviation in WWII out there.

Considering the shortage of metal, those are most likely made of wood.

The '6MU' on the chocks denotes #6 Maintenance Unit, Brize Norton.  He 162 #120086 was captured in 1945, displayed at Hyde Park in London and then moved to Brize for storage. And the chocks are almost certainly wooden. And, of course, the uniforms are RAF, not Luftwaffe!

Wait, that means this picture is made in England, not in Germany? You're really helping me out with your comment, thanks a lot!

@Noah: Indeed, England. There is more history here: http://captured-wings.wikia.com/wiki/Werk_Nr_120086 and  https://documents.techno-science.ca/documents/CASM-Aircrafthistories-HeinkelHe162A-2Volksjager.pdf

@Dave Great, great, great! Thanks!

You realize that photograph is already in color don't you?

I thought that color was a post-war invention :P

Those chocks were made out of wood; I am guessing that they were unpainted or painted a bright color, for visibility. Today, they are made out of rubber or a synthetic. Rubber grips better than wood, but the wood chocks were most likely made out of wood, to preserve the rubber for the war effort.


text:
Can I document a VOR check with a photo to comply with 91.171(d)?
I'm interested in understanding the limits of what it means to "sign" a record. While enroute for any meaningful length of time, I try to tune two VOR's and I take a photo of the G1000 NXi on my phone to ensure I always have a current VOR check. This photo contains metadata with the date and place the photo was taken, and the photo details the bearing error. The only thing i'm missing for this photo the be the "other document" I require and to avoid having to write out a random slip of paper to that will satisfy the requirements for 91.171(d) is the signature.
I understand this signature is to certify that this information is factual and that it really took place and that the legal definition of a signature has evolved a lot over the past few decades. Clearly it doesn't matter that I infer the intent of the signature portion of the rule to be to ensure that no one lies about performing the VOR test.

Is my photo sufficient because it shows that the test was actually performed?
Should I edit my photo and scribble a "signature" on it?
Maybe a live photo with a "voice signature" would work?
Or do I need to keep writing on little scraps of paper to cover myself?

Welcome to aviation.SE!

The only time this will ever come up is in an enforcement action.  I think you would meet the intent of the regulation by the picture especially if the date and time are included in the picture somehow.

Date, time and even GPS coordinates can be (and are, by default) stored in the EXIF data added to most every digital picture taken. Unfortunately, it's quite easy to edit EXIF data, so the FAA might question you on it.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Is the time spent flying in a Sailplane concidered a valid flight hour for a LAPL / PPL license?
I live in the Netherlands.
In order to get a Light aircraft pilot licence (LAPL) license 30 flight hours are demanded, A private pilot licence (PPL) requires to have 45 flight hours.
flying in a Sailplane is lower-priced then a motorized airplane in my country.
So I'm wondering if flying in a Sailplane is concidered a valid flight hour for a LAPL / PPL license.

In the UK, under the CAA (and as a CVn mentions, it may vary across EASA land, regardless of SERA), these are the rules:

The privileges of your licence will only remain valid if you have
  completed, in the last 24 months, as pilot of an aeroplane or TMG:
At least 12 hours flight time as PIC, including 12 take-offs and landings; and
Refresher training of at least 1 hour of total flight time with an instructor.

That document also explains that:

A LAPL for aeroplanes will allow you to act as pilot in command (PIC)
  on two classes of aircraft: either a single-engine piston aeroplane
  (land) or touring motor glider (TMG) with a maximum take-off mass of
  2000 kg or less, carrying a maximum of 3 passengers, with no more than
  4 persons on board.

So I think that, in the UK, you can't act as PIC on a sailplane and so can't count any hours toward your LAPL
However, TMG flying is very cheap, if you can find one for hire

I guess you are right, however, I think that flying in a sailplane will improve pilot skills and knowledge about aviation. So it doesn't hurt.

@Julian you are absolutely right.  All else being equal, a glider pilot transitioning to power almost always makes a better power pilot.


text:
Why would a pilot pick this military airport as an emergency landing airport?
Looking at this old question, a comment suggested that Skyway Airlines flight 1267 was a possible answer to that question. Out of curiosity, I looked at the NTSB link of the Nov. 1996 incident report and was very surprised to find that this flight was on its way from Milwaukee, WI (KMKE) to Indianapolis, IN (KIND) (a 206 mile flight) but diverted to Fort Campbell, KY (KHOP) which is 191 miles south-southwest of KIND.
On its way from the KIND area to KHOP, the flight missed Cincinnati KCVG (85 miles from KIND) and Louisville KSDF (96 miles from KIND) as well as numerous other smaller airports that should have been able to accommodate a Beech 1900D.
I could understand this choice of destinations if KHOP were a civilian airport that happened to be a hub/maintenance facility for Skyway Airlines, but it's a military base.
Why would a pilot go so far out of his way to land at a military airport when there were numerous other suitable airports that were significantly closer, including, presumably, KIND itself, since the flight had to pass by there to reach KHOP?

Emergency landings often dictate landing at the safest airport, not necessarily the closest one. As I said in the comments, the flight was operating IFR and had turned the window heater on presumably for anti-ice. It could be that a line of weather was preventing VFR operation at closer airports and they diverted to the closest possible VFR airport.
When encountering an emergency there are things you don't want to do. You don't want to panic to get the aircraft down if the aircraft is perfectly flyable. You don't want to make major configuration changes, especially multiple ones, quickly. Pilots are taught to "Aviate, Navigate, Communicate" in that order. As a pilot you should prioritize flying the plane, work the problem, and determine the safest place to put down.
It is possible in this case that they could have continued IFR, but if they encountered icing below the missed approach point they would have been put in a situation with icing and no visual references.
It is also possible that the pilots recognized that some electrical problem was at fault, and even without icing conditions they would want to remain VFR because they didn't trust the electrical system would continue to function throughout the IFR approach and landing.

I would wager the specific decision process behind this choice is more fully described in the NTSB's Investigation Docket for the incident. Sadly as it's an older investigation that's not available online, but it can be requested (information on that process is available at the end of the report).


text:
Is the FAA Medical Exam an IRS Medical Expense?
Does the FAA Medical Exam for a medical certificate qualify as a Medical Expense per IRS Publication 502 (thus allowing one to deduct the expense, or utilize a Health Savings Account (HSA)/Flexible Spending Account (FSA))?
One possible way to have the medical exam qualify as a medical expense would be to have a standard physical exam and the FAA medical exam together (which might also let you claim it with your health insurance).
Is this interpretation correct or consistent with your experience?

Your question title is asking about the IRS, i.e. is the medical tax-deductible? But the AME quote is about claiming the costs from your insurance company. Those are totally different things. I assume you're asking about the tax issue because that's - in theory - the same for everyone, whereas the insurance question depends on your policy so it isn't really answerable. Is that correct? If so, I suggest you remove the AME quote to avoid confusion.

Thanks. I've edited my question to reflect the primary focus (is it qualified medical expense for the IRS?) but also kept the pointer to billing insurance (for those interested).

FWIW, during the 30 years I was flying professionally, I always claimed it as a job related expense, and I was never audited.

If you're asking whether you can pay for one with an HSA or FSA, then yes, physical exams are eligible expenses, though consult with your insurance provider first if they are covered by your plan.

Physical Examination
You can include in medical expenses the amount you pay for an annual physical examination and diagnostic tests by a physician. You do not have to be ill at the time of the examination.

If you're looking to add it to a list of itemized deductions, as Pondlife explained, it generally has to be business- or workplace related.  If you are a professional pilot, then this is deductible.

What Can I Deduct?
To be deductible, a business expense must be both ordinary and necessary. An ordinary expense is one that is common and accepted in your industry. A necessary expense is one that is helpful and appropriate for your trade or business. An expense does not have to be indispensable to be considered necessary.

i would keep in mind that it's inappropriate to exclude things twice - your hsa/fsa spending is already not taxed, as contributions made to it are already deductible, so you can't reduce your taxable income further by claiming an individual expense a second time.

For those without HSA/FSAs, you would be able to deduct the physical exam as a medical expense (provided that your total medical expenditure exceeds 10% of AGI or 7.5% of AGI if you are over 65). Having an HSA/FSA means you don't have to worry whether you can deduct it or not: it would be paid for with pre-tax money already set aside for qualified medical expenses.

It looks like if you're a professional pilot then yes (it's an unavoidable expense for your job). Otherwise, no (probably, see disclaimer below):

Flights in the first three categories [including for FAA medicals] were clearly personal,
  reflecting petitioner’s family or personal obligations, personal
  finances, or love of flying

And this from AOPA:

The first thing you'll notice in these regulations is that your flight
  training expenses will be deductible only if they are trade or
  business related. Therefore, if your flying is purely personal in
  nature, you will not be able to take a tax deduction for flight
  training expenses.

Admittedly that talks about training, not medical expenses, but a medical is required unless you're a sport pilot so it seems to apply.
Disclaimer: I'm not a tax attorney, if you want an answer that will stand up in court than ask someone who is :-)

Your first link says that it could be written off as a business expense, not as a medical expense. Also, in the case you mention, petitioner claimed the medical exams as business expenses (though I suspect if he thought he had a chance he would have claimed it as medical, so you might have a point there).


text:
What is the purpose of the extra wheel on the XB-70 main gear?
This photo of the XB-70 Valkyrie's main gear shows a small wheel between the two main wheels:
Courtsey of AirPigz.com
This smaller wheel doesn't touch the ground, though I can see where it might as the side walls of the main tires compress under landing weight.
The AirPigz site promised an explanation, but none has been forthcoming (in the 4+ years since that was posted), and I couldn't find any information on it on Wikipedia, either. Does anyone have any information (real or speculative) on what it's for?

Ha! I spotted that on the video that was posted the other day, but was too lazy to ask. Thanks!

It is a sensing wheel that is part of the antiskid braking system. It contains an electronic sensor to measure the true groundspeed of the airplane. See NASA Technical Note D-4836, Landing Loads and Accelerations of the XB-70-1 Airplane (1968), bottom of page 3 of the document (page 5 on the PDF):

Wow! A quick and authoritative answer! Did you know this or is your Google-fu that good?

Call it a lucky Google.

Nice "lucky Google", then Fred! I'm sure I could have done the same, but it's good to have information gathered here on what's quickly become my favorite web-site. Thanks for the image embed, @mikeSowsun


text:
Is it possible to land a plane only with the Garmin® Synthetic Vision?
A synthetic vision system shows the pilot the position of his plane in a 3D-modelled flight environment.
It's like a 3D TAWS. It also shows the position on the runway:

As I have seen, this feature works quite good. It's sometimes a bit off the centerline, but on the runway.
It is clearly stated that this is only to be used as feature to enhance the situational awareness. And I am also quite sure that this is not authorized as primary navigation source.

But imagine: You are flying in zero visibility. You have an emergency and must land. There is no ILS. (You have done no flight planning and the bad weather surprised you)

Would it theoretically be possible to land a plane only with this synthetic vision?
Is it accurate enough?
Would the plane at least come down in one part?

Apparently yes, according to this 2008 review of the system in a Diamond DA40:

With [a safety pilot], I stayed under
  the hood for the entire flight—takeoff, en route, approach and
  landing—using the system’s visual cues. I managed to land the airplane
  without breaking anything, barely. It wasn’t pretty, but it
  demonstrated what was possible.


text:
What is the influence of drag on an airliner's block fuel consumption?
I am not sure how widely the software is used, but I am using Piano-X to investigate the influence of total aircraft drag on block fuel consumption and noticed something odd. It is my hope that someone familiar with the software can assist me.
If I decrease the total aircraft drag both the block time and fuel consumed increases, which is the opposite of what I would expect. As expected however, the block fuel/min does decrease.
Shown below is the results from the investigation for the drag systematically decreased :

I expect that the block fuel/time would decrease but don't understand how the total time and fuel will increase for decreased drag.
Is there a rational explanation for this?
The software documentation only states the following concerning drag calculation:
"Aerodynamic drag is calculated in detail (as a function of lift coefficient, Mach number, and Reynolds number) and tuned with actual lift/drag data (‘polars’) whenever these are known. You can either factor the drag uniformly using one overall value or you can adjust certain items individually."
My approach was to vary total drag, i.e. induced and parasite drag simultaneously.
I will add that the software calculated total block fuel including estimates for both taxi's and the climb-out etc. Now in taxi drag doesn't have an influence, but I doubt that this will make such a large contribution.

How do you / does Piano X change the drag? By changing the aircraft's speed or by changing the aircraft's design? From what I can see it looks like the aircraft is flying slower with the lower drag numbers. If you are using a jet / turbofan engine it would make sense that the block fuel goes up.

Why is it measuring time in meters?

That chart seems to show the time taken going up by almost a factor of 2 when the drag is cut in half. If drag is changed only because speed changes, then your COEFFICIENT of drag didn't change, just the speed did. If the program were truly changing the coefficient of drag (simulating a cleaner, less draggy aircraft), then it is not at all obvious why speed would go down like that, and it is counterintuitive that fuel burn would go UP with the cleaner aircraft.  Something else is clearly at work here.  Program bugs or misunderstood/misrepresented parameters would be my top 2 guesses.

I realize that my question is a bit vague, I was actually hoping that someone familiar with Piano-X will be able to help. Th documentation of the software is dismal. I have updated the question stating how the documentation calculates the drag.

Is it constant speed, constant range? The key is there. Are you able to plot those other parameters?

@TrebiaProject, during all the calculations the aircraft flight mission, i.e. payload and range, is kept constant.

@raptortech97, if you are clever enough to notice that , I am sure you are also clever enough to deduce that I meant 'm' for 'minutes' (-;

I contacted the makers of the software and it turns out that range was not kept constant and that was the reason for the behavior. Sort of a bug in the program.

Reason for question being asked was due to a bug in a software package

I would suspect that reducing drag in this case would be result of reducing airspeed. If your drag coefficient 1 is for a certain speed, slowing down would cause smaller fuel flow but increase flight time and hence increase total consumption.

That is not necessarily the case. Slowing down from typical cruise speeds generally gives greater fuel economy, not less, since you're usually cruising faster than max range & max endurance speeds.  Of course, if you are starting AT the speed for max range, slowing below that WILL increase fuel burned per mile traveled, but that's usually pointless.

@Ralph J Correct. But assuming any speed for any coefficient on the given graph is a little hard.

Totally agree.  Unless an expert on that particular software can address what's going on, there are far too many unknown assumptions here to make sense of the data provided.  On its face, the data makes no sense whatsoever as far as I can tell.

I contacted the makers of the software and it turns out that range was not kept constant and that was the reason for the behavior. Sort of a bug in the program. If you remove this answer I will delete the question. I think it will be better.


text:
What happens if only rudder is applied in a turn without ailerons? Do the two have to be applied together all the time?
Fresh newbie in glider flying. I was told the joystick (aileron) and the pedal (rudder) should always be applied together when making a turn. My question is, what happens if I only pedals the rudder without applying the stick (aileron)? How is it different from applying only the aileron but not the rudder? And are there times when we should apply just one of the two?

Dupe of Can some planes turn using just the rudder or without banking? Which itself is marked as a dupe of Are the functions of ailerons and rudder similar?

@TomMcW Are you saying that this answer post completely answers the question asked here? I think it clearly doesn't, and my understanding is that a question is only considered a "duplicate" if there is an answer post on another question which answers it.

@TannerSwett I’d say when you apply say left rudder the plane will bank left and turn left but you have to control the turn rate by power and rudder answers most of it. The last part is new, and maybe the question should be edited to focus on that part. But I’m the only VTC so far, so it will prob remain open anyway

@TomMcW Well, that does answer the question of "what happens if I only use the rudder and not the ailerons", but not in enough detail, if you ask me. It doesn't answer the question of "are there times when we should apply just one of the two" at all.

My question is not really dupe to the linked ones (I do appreciate the links which are helpful, too). I wanted elaborate answer to "what happens", not just yes/no or short answer without further explanation, so I'm happy and grateful with the answers given here. Also glad to see the mention of slip and skid, which were not clear to me before.

My glider training included learning how to land the glider safely with a complete loss of aileron control, or with a complete loss of rudder control (obviously, not both at the same time). It is possible to turn with ailerons only, or with rudders only, but it is extremely slow.

Hint: there is one situation where it is very important to use only the rudder while keeping the stick strictly in the middle. Many pilots died in the first decades of aviation before it was figured out.

@vsz What situation is that? Something that sounds life threatening doesn't seem like it's appropriate to tease about

@Xen2050 : I thought it was obvious to everyone that I was talking about the spin.

@vsz Seems a lot better to be clear on a site people visit for learning about aviation. And interestingly Tanner Swett's answer mentions turning stalls & spins separately too

@vsc -- "My glider training included learning how to land the glider safely with a complete loss of aileron control, or with a complete loss of rudder control (obviously, not both at the same time). It is possible to turn with ailerons only, or with rudders only, but it is extremely slow. " -- depends on the glider!  Wouldn't work in, say, a Ka-2.  "Anti-rudder" effect from ailerons is just too strong, vertical fin too small, even w/ very shallow turns you get "stuck" in a bank that you can't get out of even with lots of opposite stick.

To turn – any vehicle, not just an airplane – you need to generate corresponding centripetal force.
When you only apply rudder, the plane will turn a bit, but because unlike boat it has no keel, it will not generate much centripetal force, just fly somewhat sideways, which is called a skid. It will produce more drag this way.
Now any well designed plane has yaw-roll coupling, which will cause it to bank into the turn anyway. It is caused by the slight increase in speed of the outside wing, and the dihedral most wings have resulting in increase in angle of attack, thereby increasing lift on the outside wing. So you can make the plane turn with just the rudder, but it will be inefficient, and it will be slower as the yaw-roll coupling will take its time.
On the other hand if you apply just the aileron, the plane will bank into the turn as usual. However, there will be some slip, as the deflected ailerons cause an adverse yaw – the aileron deflected down, which is on the outside wing, causes more drag – so the plane will actually turn a little less than it should. Like skid, that will cause extra drag.
What you want to do is bank into the turn with ailerons, and use rudder to compensate the various drag asymmetries to keep the slip-skid ball centered. That will give you least drag. Different planes will need slightly different application of rudder.

There are cases where you will want to apply ailerons and rudder in different combinations (as mentioned in comments below):

On the ground you steer with rudder, while using aileron to prevent wind from tipping the plane sideways – the landing gear provides the centripetal force in this case.
During cross-wind landing, you need to align the plane with the runway, but as it flies relative to the air, it tends to drift sideways. You need to make it fly sideways relative to the air instead by banking into the wind while using opposite rudder to prevent turning.
The same combination applies during cross-wind take off, aileron into the wind to prevent wind from tipping you over sideways and rudder to compensate for weathervaning.
You can also use bank with opposite rudder to increase drag if you are too high and fast on the approach – this is called forward slip. Just don't let the airspeed decay too much, because stall in slip or skid is more likely to result in a spin.

Thanks. How about designing gliders (or airplanes) such that the aileron and the rudder are controlled together and move simultaneously, since they need to do so anyway?

@gadfly There are airplanes that are designed like that. One problem with designing a glider like that is that on the ground, you need to be able to steer (using the rudder) and keep the wings level (using the ailerons) independently. Another problem is that it's useful for a glider to be able to deliberately fly uncoordinated, in order to lose altitude.

@gadfly One airplane that has no rudder pedals and interconnected aileron/rudder is the Ercoupe. https://www.aopa.org/go-fly/aircraft-and-ownership/aircraft-fact-sheets/erco-ercoupe

@gadfly A properly coordinated turn requires the right amount of rudder for a given amount of aileron, which may not always be in fixed proportion, or even a fixed curve (as might be accomodated via a cam). The best solution is to leave the controls independent and allow the pilot to center the string (or ball), or not (if performing a sideslip, which can require opposed control inputs).

@TannerSwett : Another problem with the glider is having to land with a strong crosswind. You can't just fly around waiting for a better opportunity, if you start the landing, you are committed. And in a strong crosswind you need very different aileron and rudder movements to land without breaking your wing. (Also, I don't think one could use thermals with the huge turning radius if you use only the rudder for turning)

An example of where a slip was helpful is the Gimli Glider where a 767 ran out of fuel and had to make an unpowered landing. They were coming in too high and used a forward slip to intentionally increase drag so they could make the runway.

If you apply (say) right rudder without any aileron input, the immediate effect is that the aircraft will yaw: the nose will swing to the right.
Note that this does not mean that the aircraft will turn! In the first moments after the aircraft yaws, the nose is now pointed off to the right while the aircraft continues to travel in its original direction. What the aircraft will "feel" is that the air is now blowing on it from the front left instead of head-on. We say that the aircraft is slipping to the left, because its track (the direction in which it is traveling) is to the left of its heading (the direction that the nose is pointing).
So, you have stepped on the right rudder pedal, causing the aircraft to yaw right and slip left. This has the following consequences:

Your instruments will indicate a slip to the left. The yaw string will indicate this by moving to the right (meaning that the "head" of the yaw string will point to the left.) The inclinometer (the "ball in a tube") will indicate this by moving to the left. And your inner ear and body will indicate this by telling you that the centrifugal force from the turn is pushing you to the left.
Drag will increase dramatically. In a glider, this means you will either slow down or descend more quickly, depending on what you do with the pitch. In an airplane, the same thing will happen unless you add engine power.
Body lift. Since the oncoming air is blowing on the aircraft from the left, this will push on the aircraft and turn it to the right. (This pushing force is what causes the inclinometer to move, and your body and inner ear to feel the sensations I mentioned above.) This is, of course, not the "normal" way to turn an aircraft.
Dihedral effect. The air blowing from the left will cause the aircraft to roll right. (Here's one way to think of this. The tips of the wings are "high up" compared to the rest of the aircraft. So when there's air coming from the left, it will push the wingtips to the right, causing the aircraft to roll right.)

Slipping is the same thing as being in uncoordinated flight. Being in uncoordinated flight presents two problems. The first problem is that it creates a lot of drag. (Sometimes you want drag, of course.) The second problem is that if the aircraft stalls while in uncoordinated flight, the downwind wing will stall first, which can produce a snap roll or a spin. Snap rolls and spins can both be recovered from, but if they happen close to the ground, the results can be fatal.
Using just the rudder and not the ailerons has some uses:

When you recover from a turning stall, after regaining speed, you should level the wings by leaving the ailerons alone and applying rudder towards the sky. We use the rudder here because using the ailerons can exacerbate a stall.
The procedure for recovering from a spin involves using rudder opposite the spin while leaving the ailerons neutral.

Using the rudder along with opposite aileron will produce a straight slip, which is useful for deliberately losing altitude  or for landing in a crosswind.
As for applying ailerons without rudder: applying (say) left aileron without rudder will have a similar effect to applying right rudder without aileron. In fact, left aileron without rudder may even cause the aircraft to yaw to the right. (This is called adverse yaw.) As far as I know, the only difference is that the aircraft will bank to the left, which will ultimately result in a left turn instead of a right turn. The dihedral effect will still happen, just as if you had given right rudder; since you're pushing the stick to the left, this means that the dihedral effect is resisting your attempt to bank left.
Finally, on the ground, pretty much all of what I just said goes out the window. On the ground, the rudder is used to steer the aircraft and the ailerons are used to keep the wings level (if necessary). Unlike in the air, while you're on the ground, yawing the aircraft will cause it to turn immediately, thanks to the wheel or wheels.

Minor Q: "slipping to the left" vs "slipping to port" ?  What's more common?

"Left" is definitely far more common. I almost never hear "port" and "starboard" used in reference to aircraft.

Thank you for the elaborate answer. I would not be satisfied with a short answer.

If initiated by rudder, this is correctly called "skidding" rather than "slipping."   "Slip" is when you use ailerons without rudder, or initiated crossed control maneuver with bank -- skid is rudder initiated cross control or rudder without aileron.

So if you upped your engine power to correct for the drag?

@Joshua Sometimes I forget that some aircraft have engines! I just edited my answer to mention that yes, you can counteract drag by adding engine power.

I was more interested in what the now off-center engine force was going to do (because the hull isn't pointing in the direction of travel) but ...

That's a great question and I don't have a great answer. Just going by my knowledge of physics, if you add power while you're in a slip, your forward speed should increase and your lateral speed should stay the same. I don't know what the consequences of this are.

"Note that this does not mean that the aircraft will turn!" I think "turn" is generally understood to refer to a change in heading, not track.

With just rudder, your tail will kick out to one side, and you'll kind of skid around a turn in a really draggy manner.
With just aileron, the plane will turn but the tail will droop to the inside of the turn, also a kind of draggy maneuver.
When you coordinate both, the plane banks and the tail moves the fuselage around the turn in a nice smooth circle.

Thanks. So would it be right to say that the two are always applied together, without exception?

@gadfly Not necessarily. If you are flying an instrument approach and need to make small corrections to keep the needle centered, you might use just the rudder to get back on course. It’s usually for changes in heading of less than 5°.

@gadfly Another exception to the rule of using the aileron and rudder together in a coordinated turn is when you user a slip to counteract a crosswind when landing. Also, light aircraft, especially those without flaps, might use a slip to produce steeper glidepath.

Even those of us With flaps use slip for steeper glidepath, to lose altitude, or to slow down some.

Aircraft can be designed (with relatively large dihedral) to fly without ailerons, using only rudder (and the roll coupling that the dihedral produces) for both directional and roll control.  Without this designed-in coupling, rudder-only input generally results mainly in a skid, with only a small bank (and as noted in other answers, a very inefficient turn).
Many radio control trainers have been designed for rudder/elevator control; I owned and flew one many years ago that I could roll and fly in prolonged inverted attitude, even do outside snaps to a tumble that resembled a Lomczevak (and recover into level flight after two somersaults).  Two-axis control is also common in R/C sailplanes -- Gentle Lady, Olympic 2 and Olympic 650, Two by Six, etc.
It's relatively rare for human-carrying aircraft to lack ailerons, though not unheard of (the Pou de Ciel homebuilt family mostly have no independent roll control), because in general, it's more useful for safety reasons to have direct roll control (perhaps with coupled rudder like the original Ercoupe) than to depend on high dihedral for roll -- crosswind landings, for instance, are far easier in an Ercoupe than a Pou.  If you need a rapid roll correction, ailerons will give it; rudder and dihedral won't (there's some lag in roll because the dihedral requires the skid before it raises the forward/outside wing).

The controls all have primary effects of the control, and followon effects of the control. See Jeremy Pratt, section "Effects of Controls". https://stepbystepppl.wordpress.com/2013/07/02/ppl-exercise-4i-the-effects-of-control-surface/
If you are in level cruise (and you are in a reasonably "normal" airplane like a C-150) and give a rudder input, as many have pointed out in their answers, the primary effect is a yaw effect.
But, if you gently continue the yaw input, the yaw has a followon effect once the airplane settles into the new unbalanced attitude. The followon effect is a bank in the direction of yaw. The result therefore, if you are very gentle, is a coordinated turn.
My venerable mentor taught me to learn coordinated flight by flying exactly this way - in the cruise, fully trimmed, hands free and steering by gentle rudder inputs alone.
The major caveat is that every airframe is unique and you need to learn the particulars of yours. In my case, this of course was low level powered flight in a Cessna 150.

Aircraft work in three dimensions, not two, so the rudder is not used to 'steer' an aircraft they way you would a boat or a car. The three motions available to you are called pitch, roll and yaw and are controlled by the elevator, ailerons and rudder. All three are used together to change the direction an aircraft.
To change direction, you have to introduce a force that pushes you from where you are going to where you want to be. To go from straight and level due north to straight and level due west requires all three controls and a careful balancing act. Banking the wings left will change the lift vector from vertical into the corner pushing you in the direction you want to go, however with less lift, the nose will go down so you pitch up to keep the nose level, which also pulls you further into the turn. However, being banked means the plane will side slip can feel uncomfortable so you use the rudder to adjust the yaw to compensate. All of this pushing against the air to make the turn also robs you of energy that needs to be replaced, causing you to slow down or go down.
You can turn without the rudder and normally do, but you can not turn with the rudder only.


text:
What is the wing span in aspect ratio's calculation?
We all know that the aspect ratio of an airplane is calculate as:
$$AR=\frac{b^2}{S}$$
Where:
$AR=$ Aspect ratio
$b=$ Wingspan
$S=$ Wing's area
In an airplane, wing span is the distance from tip to tip of the wing. That means the fuselage also calculated. Below are two picture of two different kind of airplane: Boeing and Cessna. As you see, the wing is placed in different position. The Boeing is below of the fuselage so it look like integrated to the wings. But the Cessna is placed on top of the fuselage, make the fuselage is like the separator of the two wings rather the integrator.

But in another time, what we call as wing is a wings part that are attached to the fuselage, both the left and the right side. The picture below shows what the wing span is. The fuselage itself is NOT considered as part of wing.

To avoid miscalculation, I would like to know what is exactly b in that equation. For this case, just consider the wing shape is fully rectangle. Appreciate if with reference.

In aeroplanes, the wing is actually one complete, uninterrupted structure, and the fuselage has holes in it to allow the wing to be placed. The middle part of the wing has the highest bending moments, a cut in the wing structure here would pose large structural engineering problems. You can attach two half wings like in the old picture, but only to a centre structure that can absorb all bending moments and torques. The wing central structure, part of the wing.
In birds, the situation is different. There are clearly two wings on a bird, both hinged and independently moveable via muscles.
But for both aeroplanes and birds, the wingspan is from wingtip to wingtip.

Quote: "In aeroplanes, the wing is actually one complete, uninterrupted structure." I can understand this. In the very beginning, Wright brother had their plane with an integrated wing. But if so, with the modern aero plane like the Boeing and Cessna as I mentioned above, what about the front side and aft side of the fuselage, shouldn't we need to consider them as wing as well?

We could. Structurally it would make no sense, aerodynamically the situation creates interference problems either way. Also,, in the times when aerodynamic computations were researched (no computers yet!) the wings on biplanes were not interrupted by the fuselage. So the wing span from tip to tip was chosen.

b is the width of the wing's projection on the horizontal plane, from tip to tip. The wing's vertical position on the fuselage does not influence b. This may or may not include winglets and ideally should be clarified when b is specified.
Calling one half of the wing a wing is sloppy use of language. Ideally, a wing half should be called that, and not be confused with a wing.

Appreciated if you may elaborate more detail. To be honest, still very hard for me to get the point, especially whit this: Where the wing is relative to the fuselage does not influence b.

@AirCraftLover, whether the wing is high or low on the fuselage doesn't matter for such calculations. The wing is considered as a whole. In essence, simply by convention. If you think of it, the 'Cessna' (high) wing is even more 'integrated' than the Boeing one, given that it is the top surface that creates the most lift, so to speak.

@Zeus, do you have a reference for that "integrated calculation", please? Appreciated if you have.

@AirCraftLover, not at hand. It is a general (academic) knowledge that high wing normally has less negative (and in fact, often positive) wing/fuselage interference than other designs. Explaining why is worth a separate question. But anyway, this is not directly relevant to your question: in all cases (in aircraft design and aerodynamics) "wing span" is defined as the total span, tip to tip, including the fuselage or whatever is in between.

In calculating the area, we know that the wing and the fuselage are not flat, should I project the surface area seen from above parallel (or in line) to gravity? Or should I do separate calculus integral do find the surface area of all the airplane's parts?

Another thing, how about the fuselage overall and the tail parts (elevators and horizontal stabilizers), are they included to the surface area? Thank in advance for your kind to comment.

@AirCraftLover: No, but the wing area is extended into the fuselage. There are several ways for doing this; read here for more.

Dear @PeterKämpf, apologize that I have to ask you again. Not to debate you, but I do really confuse about this wing area. As I read here in the NASA web page, it is not mentioned that fuselage is calculated as part of the wing. Do you have please reference to it, moreover if any example of the calculation.

@AirCraftLover: First, posts to my answers do not need my name; they will reach me even without it. Next, maybe you trust Researchgate more than me. The NASA GRC pages are for kids and oversimplify too much. You could also read a lot of other sites - you will find out that my answers are correct.

If the fuselage part between the left and right wings considered as part of the wing area, then how about another fuselage part front and aft side of the wings, shouldn't we need to consider them as wing area as well?

@AirCraftLover Definitely not. If we would, what would be the distinction between wing and the whole airplane? The wing part inside the fuselage is somewhat arbitrarily defined, but never includes fuselage sections ahead or aft of the wing.

I read that link, I just need to think carefully before I write my comment, quite confusing as the Airbus and the Boeing way is different. I can accept the Airbus' way but not the Boeing way.

I posted my post here (https://aviation.stackexchange.com/questions/101114/how-the-wing-area-of-an-airplane-is-determined) about how to determine wing area of an airplane. This wing area is very crucial as it related to its safety besides many more things, such as minimum speed and so on. Can you give a simple guideline to determine that wing area? Or, should I just follow the Airbus' method?

All the reference quantities for a wing are just that -- reference.  They typically refer to a simplified and idealized trapezoidal wing similar (but not exact) in planform to the real wing.
Transport wings often have trailing edge extensions (Yehudi) and winglets that are omitted from the reference wing.
The reference wing continues to the aircraft center line -- it is not broken up by the fuselage.
The reference wing is used to calculate Sref, b, AR, and other quantities.

What do you mean with Sref?

Reference wing area, used for calculations.


text:
How many B747s SCA does NASA have?
How many B747s that used as Shuttle Carrier Aircraft (SCA) does NASA have?
Please include:

The number of B747 SCAs that are still in service.
The number of B747 SCAs that have already been retired (if any).
The number of B747 SCAs still to be delivered by Boeing (if any).

The shuttle was retired in 2011. Why would you expect that Boeing would still be delivering 747s to NASA in 2019?

Your title asks about NASA's 747s, but your question refers only to Shuttle Carrier Aircraft. To my knowledge, NASA has at least one 747 that was never involved in the Shuttle program. It's used as a flying observatory. Are you interested in this and any other non-shuttle aircraft?

@ReddHerring, even I didn't mentioned B747 in the question body, people should understand that B747 is the only aircraft capable to carry the shuttle aircraft, plus one ever build Antonov AN-225, Mriya. But however, AN-225 is belong to Ukraine, not US.

@egid, even the shuttle is retired, but will come new shuttle. I think, US will never stop exploring space.

I'm confused. There are no plans for a new NASA spaceplane. What are you referring to?

@AirCraftLover The 747 and the AN-225 are NOT the only aircraft capable of carrying a shuttle, those are the aircraft chosen, The C-5 was a contender as well. Also STOP CHANGING YOUR QUESTION. Originally you asked how many 747s NASA had in it's possession. That's why I mentioned SOFIA along with the SCAs in the first place. It makes my answer off-topic.

Also mentionned in Could the Shuttle Carrier Aircraft do a transatlantic flight with a Space Shuttle orbiter on its back? NASA 905 obtained from American Airlines in 1974; NASA 911 obtained from Japan Airlines in 1989. They were B747-100 and B747-100SR, modified by Boeing into their SCA configuration.

NASA had two Shuttle Carrier Aircraft. One was a modified Boeing 747-123. The other a 747-100SR-46. The SCAs were used to ferry space shuttle orbiters from landing sites back to the launch complex at the Kennedy Space Center and also to and from other locations too distant for the orbiters to be delivered by ground transportation.
NASA 905, a Boeing 747-123 model has been retired. It is on display at Space Center Houston.
NASA 911 was the second SCA, a Boeing 747-100SR-46 version. It was retired and is on public display at the Joe Davies Heritage Airpark in Palmdale, California.
In all, NASA has been in possession of three Boeing 747s. The third was not an SCA, but an airborne observatory known as the Airborne observatory, Stratospheric Observatory for Infrared Astronomy (SOFIA). The aircraft is a Boeing 747SP-21 which is housed at the Armstrong Flight Research Center.
Currently, NASA lacks an active aircraft capable of carrying a Space Shuttle.

https://en.wikipedia.org/wiki/List_of_NASA_aircraft
https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-013-DFRC.html

I just googled N905NA, and it's definitely not an SP.

If I checked over the net many pictures of the B747 SCA, seems like SOFIA is not SCA. There is no support on the back where the shuttle aircraft will be mounted.

As already indicated by that Wikipedia, SOFIA is a Stratospheric Observatory for Infrared Astronomy, not a shuttle carrier. Here is video of how it work: https://www.youtube.com/watch?v=Y18NYGlIukM.

Wow @AirCraftLover, I indicated there was only two SCAs, but NASA was in possession of three 747s and posted a list so that others could see the inventory of NASA aircraft.

Oh I see. Actually my focus is to the vertical stabilizer why is is required, as I was confused about the B747 SOFIA, I asked it in another question. But now it is confirmed that SOFIA is not an SCA.

@AirCraftLover Your question title reads how many 747s NASA has, not how many SCAs NASA has.

My question become very bad due to avoided complain as "too broad". Actually I was confused about the SOFIA, but I didn't know how to describe it as I saw also NASA's logo on the vertical stabilizer, but the stabilizer is different than another which there are another two additional vertical stabilizers at the tip of the elevators. But my effort to make simple question was credited very bad. So clearly, this question is an intermediate question to ask the vertical stabilizer I asked in the comment below.

Here it is: https://aviation.stackexchange.com/questions/60708/why-do-the-b747-of-nasa-shuttle-carrier-aircrafts-scas-need-additional-vertica